1. Field of the Invention
The field of the present invention relates to location services. More particularly, the field of the present invention relates to locating a mobile device by noting times of arrival of signals that travel between the mobile device and a plurality of transmitters or receivers located at known positions, and computing a location based on the noted times of arrival.
2. Background
Recently, the FCC mandated the introduction of location services that can accurately locate wireless subscribers on all wireless networks. Two previously proposed approaches for implementing such location services are the “uplink” approach and the “downlink” approach.
FIG. 1 depicts a conventional uplink approach, where the mobile device 10 (also referred to as a “handset” or “remote terminal”) that is to be located sends out a signal such as a random access channel (RACH) burst. The time of arrival (TOA) of the signal is determined at each of a plurality of location measurement units (LMUs) 15–17, together with an associated quality indicator σ. This σ is an estimate of the standard deviation of the TOA measurement. Each of these noted TOA measurements and σs is then sent to a mobile location center (MLC) 19, which is a special purpose processor. The MLC 19 then uses conventional algorithms, which are well known to those skilled in the art, to determine the location of the mobile device 10 based on the TOA measurements and σ determinations made by the LMUs 15–17 and the known location of the LMUs.
One suitable conventional location algorithm uses a Taylor search to locate the intersection of two or more hyperbolas. Details of a such an algorithm can be found in “Statistical Theory of Passive Location Systems” by D. J. Torrieri, IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-20, No. 2, March 1984, which is incorporated herein by reference and is an indication of the existing level of skill in the art. This algorithm locates the mobile device by finding the intersection of two or more hyperbolas determined from three or more stations. Ordinarily, this type of location algorithm is applied to three or more TOA measurements (which are used to determine the distance from the mobile device to the LMU based on the speed c of the signal), the associated σs, and the known locations of each LMU.
FIG. 2 depicts a conventional downlink approach, where each of a plurality of base stations (BTSs) 21–23 sends a signal to the mobile device 20 to be located, and the mobile device 20 determines the TOA and σ of each of these signals. These TOA measurements and σs are then transmitted to an MLC 29, which implements a conventional algorithm to determine the mobile device's location based on the TOA measurements and the σs, similar to the algorithm in the uplink type systems. Alternatively, if sufficient processing power is available in the handset 20, the algorithm may be implemented in the handset. When the transmission frames of the BTSs 21–23 are not synchronized, the downlink algorithms are somewhat more complex than the uplink algorithms because the MLC must obtain the relative time difference between each BTS transmission to calculate a location. This relative time information can be obtained using LMUs 25, 26 located at known locations to measure the TOAs of the signals from the BTSs 21–23, in a conventional manner.
One advantage of downlink location systems over uplink systems is that extra downlink handsets can be added onto a communication network without adding extra capacity to the network. Another advantage of downlink systems is that the required LMU density is lower for downlink systems, because downlink LMUs are only needed to synchronize the frames between the BTSs, and not to make TOA measurements of signals arriving from multiple handsets. But in order to implement downlink location services, TOA measurements must be made in the handset. Unfortunately, most handsets already released in the field (hereinafter “legacy  handsets”) cannot implement the TOA measurements required for downlink location estimation. As a result, these legacy handsets cannot be located by a downlink system. This inability to locate the vast majority of existing handsets is a major shortcoming of downlink systems.
Another shortcoming of conventional locations systems is that each conventional system only works using a single communication protocol, and cannot locate handsets that use different communication protocols. For example, a location system designed to locate TDMA handsets will not be able to locate handsets that communicate using GSM or AMPS. Because location services must be provided for all types of handsets, the conventional approach for supporting all communication protocols would involve a separate location system for each protocol, and a correspondingly expensive infrastructure.
In addition to the high monetary cost of providing a separate location service infrastructure for each of the various technologies, there may also be high costs from a community relations standpoint. Some homeowners try to block carriers' plans to install antenna towers near their homes. As a result, a carrier may have to spend upwards of $50,000 to obtain approval for a single antenna site, and may have to wait a long time to obtain the required administrative approval. When separate infrastructures are installed for each technology, these problems may be encountered over and over with each new installation.
The inventors have recognized a need to obtain the benefits of downlink location services while avoiding the shortcomings of downlink.
The inventors have also recognized a need to provide location services for handsets that use many different communication protocols, without using a prohibitively expensive infrastructure.